Cisco Catalyst 4500 Series - Traffic and Network Protocol Disruption During ISSU in a VSS

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Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 5 Configuring Virtual Switching Systems
In-Service Software Upgrade (ISSU) on a VSS
Figure 5-9 Upgrading VSS System
Traffic and Network Protocol Disruption During ISSU in a VSS
Figure 5-9indicates that both switches in a VSS reboot at some point during the upgrade process.
When a switch reboots, all the network links that terminate on that switch undergo a link-down event.
This means that network devices that are connected to the switch that is rebooting will observe a
disruption in service, unless the connection is over an MEC that contains at least one link that terminates
Active
(version X)
VSL
Standby
(version X)
Active
(version X)
VSL
(Reboot)
Active
(version X)
VSL
Stateful Switchover
Standby
(version Y)
(Reboot)
VSL
Active
(version Y)
Standby
(version X)
VSL
Active
(version Y)
(Reboot)
VSL
Active
(version Y)
Standby
(version Y)
VSL
Active
(version Y)
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Note that at any given instant, at least one of the switches is Active. The Active
switch is in operation, i.e. continues to forward traffic and participate in network
control protocols throughout the duration of the upgrade operation.